1. Field of the Invention
This invention relates to fiber optical waveguides comprising a silica cladding surrounding a core, and, in particular, to multiply coated metallic clad optical waveguides employing as a first metallic cladding on the silica cladding a metal or alloy coating that does not react with the silica.
2. Description of the Prior Art
The current state-of-the-art of doped silica fiber optical waveguides has progressed to the point where low loss (below 5 dB/km) is becoming routine and the major technical uncertainty which may yet determine the success or failure of this emerging technology relates to packaging of the fiber into a cable structure that will protect the silica from hostile elements that can cause it to break. The fragility of glass fibers is well-known and it is the main reason why some early exploratory systems used bundles of silca fibers rather than single strands in their optical data links.
For many applications, the solution to the problem requires the strengthening of the individual fibers. When long-length (1 km or greater), high-tensile strength fibers are available, communication and data links can be made with light weight single strand fibers rather than heavily armored cables or with bundles.
Optical waveguides of the type discussed above are described in greater detail in U.S. Pat. Nos. 3,434,774, 3,778,132, 3,788,827 and 3,806,224. A considerable effort has been expended on coating such fiber optical waveguides with organic materials such as thermoplastics and ultraviolet-cured polymers. These materials are satisfactory for a short time, but they do not form a hermetic seal. Eventually, they will pass contaminants such as moisture which will attack the glass surface and weaken the fiber.
Metal coated glass fibers for optical waveguides have been disclosed; see, e.g., U.S. Pat. Nos. 3,778,132, 3,788,827 and 4,089,585 and Belgian Pat. No. 858,179. However, problems are often encountered employing single metallic coatings on optical fibers. Either pin holes in the coating are obtained during deposition of the metal coating on the optical fiber or, as set forth in the above-mentioned patent application Ser. No. 196,955, filed Oct. 14, 1980 many metals and alloys react with the silica, causing a degradation of strength over the long term.